In optical communication systems, messages are transmitted by carrier waves of optical frequencies that are generated by sources such as lasers or light-emitting diodes. There is much current interest in such optical communication systems because they offer several advantages over conventional communication systems, such as a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages. One such means for conducting or guiding waves of optical frequencies from one point to another is called an "optical waveguide." The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is totally reflected at the boundary with the surrounding medium, thus producing a guiding effect.
Certain electro-optical materials are very attractive for this application since they make it possible to achieve electrical control and high-speed operation in light propagating structures. The use of lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3) crystals for such purposes is well-known in the art and is disclosed, for example, in an article entitled "Integrated Optics and New Wave Phenomenon in Optical Waveguides," P. K. Tien, in Reviews of Modern Physics, Vol. 49, No. 2, (1977) pp. 361-420. These latter materials have large electro-optic and acousto-optic coefficients which are desirable to provide control of light propagation in the optical waveguide. Many different types of active channel waveguide devices using these materials have been used in a variety of modulators and switches which are compatible with single-mode optical fibers.
Various methods of forming high refractive index waveguides in LiNbO.sub.3 and LiTaO.sub.3 have been used in the art. They include: epitaxial growth by sputtering, epitaxial growth by melting, lithium oxide (Li.sub.2 O) out-diffusion, and transition metal in-diffusion. Epitaxial growth by sputtering often leads to films with high losses and poor electro-optical properties. In epitaxial growth by melting, the film thickness cannot be easily controlled. The Li.sub.2 O out-diffusion process generates a film which can support only TE polarization waves (polarization parallel to the surface of the waveguide structure) propagating along the X or Y axes.
The in-diffusion of a transition metal, such as titanium, nickel, or vanadium, into LiNbO.sub.3 or LiTaO.sub.3 offers a promising technique to produce planar as well as channel waveguide structures. However, a serious problem arises with this latter approach because at the high temperature required for metal in-diffusion, loosely bound Li.sub.2 O diffuses out from the surface of the crystal structure. As a result of this Li.sub.2 O out-diffusion, a Li.sub.2 O-deficient planar waveguide layer is formed in both the LiNbO.sub.3 and the LiTaO.sub.3 crystals in addition to the waveguides formed by metal in-diffusion. The out-diffusion waveguide can confine TE polarization waves propagating along the X-axis on a Y-cut wafer (or the Y-axis on an X-cut wafer) in an undesirable manner. (A Y-cut wafer is a wafer cut perpendicular to the Y-axis of the crystal. For a more detailed description of crystal cutting, refer to "Standards on Piezoelectric Crystals, 1949," Proceedings of the Institute of Radio Engineers, pages 1378-1395, Dec. 1949.) In a channel waveguide device, a planar out-diffusion waveguide introduces excessive cross-talk between guided modes from two adjacent waveguides. Cross-talk presents particular difficulties when trying to achieve compatibility between a fiber optic communications link and optical channel waveguide switches/modulators. The planar index increase in the C-axis caused by the out-diffusion of Li.sub.2 O limits the implementation of the optical switches to TM modes only (i.e., polarization perpendicular to the surface of the waveguide structure). In addition, in an end-butt coupling configuration between a single mode optical fiber and a channel waveguide, a large portion of the optical energy goes to the unwanted out-diffusion modes, which are readily excited by the optical fiber input, and thus the coupling to the channel waveguide is effectively diminished. It is the alleviation of these various problems caused by Li.sub.2 O out-diffusion to which the present invention is directed.
The cause of the out-diffusion of Li.sub.2 O from LiNbO.sub.3 and LiTaO.sub.3 crystals is inherent in the particular structure of these crystals. It is well known that LiNbO.sub.3 and LiTaO.sub.3 crystals can be grown in a slightly non-stoichiometric form, (Li.sub.2 O).sub.v (M.sub.2 O.sub.5).sub.1-v where M may be Nb or Ta and v ranges from 0.48 to 0.50. At the high temperature (850.degree. C. to 1200.degree. C.) required for the in-diffusion of transition metal ions in order to form a waveguide in LiNbO.sub.3 and LiTaO.sub.3 crystals, the loosely bound Li.sub.2 O diffuses out from the surface of the crystal. It is known experimentally that for a small change of v in LiNbO.sub.3 and LiTaO.sub.3, the ordinary refractive index remains unchanged while the extraordinary refractive index (along the C-axis) increases approximately linearly as v decreases. The reduction in the Li.sub.2 O concentration at the surface of the crystal due to out-diffusion thus forms a high-index layer which traps optical beams in the resulting waveguide structure.
It has been reported by W. Phillips and J. M. Hammer in the Journal of Electronic Materials, Vol. 4, p. 549, 1975 that Li enrichment at the surface can be achieved by annealing the substrates in Li.sub.2 CO.sub.3 at 550.degree.-600.degree. C. for a period of about 60 hours. In experiments where diffused lithium-niobate-tantalate waveguides were formed, all the LiTaO.sub.3 wafers were treated with this Phillips et al process before polishing. After the annealing treatment, the substrates became dark brown, were found hard to polish, and were more susceptible to cracking. The brownish color can be bleached out during the high-temperature metal diffusion. Although a reduction in waveguide loss was noted, this Li.sub.2 CO.sub.3 powder treatment failed to prevent the formation of out-diffusion waveguides. We have also tried the Li.sub.2 CO.sub.3 annealing of Ti-diffused waveguides after diffusion, but after 120 hours of annealing at 600.degree. C. in a flowing oxygen atmosphere, the Li.sub.2 O out-diffusion waveguides persisted. The present invention seeks to overcome the disadvantages of the prior art processes for eliminating Li.sub.2 O out-diffusion and to more effectively accomplish the suppression of Li.sub.2 O out-diffusion.